Basal Iron Loss

Because basal iron losses are due to cell exfoliation, these losses are relative to interior body surfaces, totaling an estimated 14 mg/kg body weight/day, and are approximately 0.8mg/day for nonmenstruating women and 1.0 mg/day for men. Basal losses in infants and children have not been directly determined and are estimated from data available on adult men. Basal losses are reduced in people with iron deficiency and increased in people with iron overload. The absorbed iron requirement for adult men and nonmenstruating women is based on these obligate iron losses.

Infancy and Childhood

The iron content of a newborn infant is approximately 75 mg/kg body weight, and much of this iron is found in hemoglobin. The body iron of the newborn is derived from maternal-fetal iron transfer, 80% of which occurs during the third trimester of pregnancy. Preterm infants, with less opportunity to establish iron stores, have a substantially reduced endowment of body iron at birth than term infants.

During the first 2 months of life, there is a physiologic shift of body iron from hemoglobin to iron stores. For the first 6 months of life, the iron requirement of a term infant is satisfied by storage iron and breast milk iron, which is present in low concentrations but is highly bioavailable (50-100%) to the infant. However, by 6 months of age in term infants, and even earlier in preterm infants, iron intake and body stores become insufficient to meet the demands for growth (expanding erythrocyte mass and growth of body tissues), such that negative iron balance will ensue at this time without the introduction of iron supplements or iron-rich weaning foods.

A full-term infant almost doubles its body iron content and triples its body weight in the first year of life. Although growth continues through childhood, the rate of growth declines following the first year of life. Similarly, the requirement for iron expressed per kilogram body weight declines through childhood from a high of 0.10 mg/kg in the first 6 months to 0.03 mg/kg/d by 7-10 years of age until increasing again during the adolescent growth spurt. Throughout the period of growth, the iron concentration of the diet of infants and children must be greater than that of an adult man in order to achieve iron balance.

Adolescence

Adolescents have very high iron requirements, and the iron demand of individual children during periods of rapid growth is highly variable and may exceed mean estimated requirements. Boys going through puberty experience a large increase in erythrocyte mass and hemoglobin concentration. The growth spurt in adolescent girls usually occurs in early adolescence before menarche, but growth continues postmenarche at a slower rate. The addition of menstrual iron loss to the iron demand for growth leads to particularly high iron requirements for postmenarchal adolescent girls.

Menstruation

Although the quantity of menstrual blood loss is fairly constant across time for an individual, it varies considerably from woman to woman. The mean menstrual iron loss is 0.56 mg/day when averaged over a monthly cycle. However, menstrual blood losses are highly skewed so that a small proportion of women have heavy losses. In 10% of women, menstrual iron loss exceeds 1.47 mg/day and in 5% it exceeds 2.04 mg/day. Therefore, the daily iron requirement for menstruating women is set quite high to cover the iron needs of most of the population. Menstrual blood loss is decreased by oral contraceptives but increased by intrauterine devices. However, recent progesterone-releasing versions of the device lead to decreased menstrual blood loss or amenorrhea.

Pregnancy and Lactation

The body's iron needs during pregnancy are very high despite the cessation of menstruation during this period. Demand for iron comes primarily from the expansion of the red blood cell mass (450 mg), the fetus (270 mg), the placenta and cord (90 mg), and blood loss at parturition (150 mg). However, the requirement for iron is not spread evenly over the course of pregnancy, as depicted in Figure 2, with iron requirements actually reduced in the first trimester because menstrual blood loss is

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Figure 2 The discrepancy between iron requirements and availability of iron from dietary absorption in pregnant women beyond 20 weeks of gestation. The resulting iron deficit is maintained as pregnancy progresses into the second and third trimesters. (Reproduced with permission from the Food and Agriculture Organization of the United Nations (2001) Iron. In Human Vitamin and Mineral Requirements: Report of a Joint FAO/WHO Expert Consultation, Bangkok, Thailand, pp. 195-221. Rome: FAO.)

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Figure 2 The discrepancy between iron requirements and availability of iron from dietary absorption in pregnant women beyond 20 weeks of gestation. The resulting iron deficit is maintained as pregnancy progresses into the second and third trimesters. (Reproduced with permission from the Food and Agriculture Organization of the United Nations (2001) Iron. In Human Vitamin and Mineral Requirements: Report of a Joint FAO/WHO Expert Consultation, Bangkok, Thailand, pp. 195-221. Rome: FAO.)

absent and fetal demand for iron is negligible. Iron requirements increase dramatically through the second and third trimesters to support expansion of maternal red blood cell mass and fetal growth. The maternal red cell mass expands approximately 35% in the second and third trimesters to meet increased maternal oxygen needs. When iron deficiency is present, the expansion of the red cell mass is compromised, resulting in anemia. Furthermore, an expansion of the plasma fluid that is proportionately greater than that of the red cell mass results in a physiologic anemia attributable to hemodilution.

To attempt to meet iron requirements during pregnancy, iron absorption becomes more efficient in the second and third trimesters. Iron absorption nearly doubles in the second trimester and can increase up to four times in the third trimester. Despite this dramatic increase in iron absorption, it is virtually impossible for pregnant women to acquire sufficient iron through diet alone because of the concurrent increase in iron requirements during the latter half of pregnancy (Figure 2).

There is also an iron cost of lactation to women of approximately 0.3mg/day as iron is lost in breast-milk. However, this is compensated by the absence of menstrual iron losses and the gain in iron stores achieved when much of the iron previously invested in expansion of the red cell mass is recovered postpartum.

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